Extreme ultraviolet (EUV) lithography requires an intense photon source at a wavelength in the region of 13 nm (nanometers) in order to image a pattern with circuit information onto a substrate that will carry an integrated circuit. Such soft X-ray or extreme ultraviolet photons can be generated in a hot plasma in which a chosen atomic species is multiply ionized. Examples relevant to lithography are the xenon plasma containing Xe10+ and the tin plasma containing states Sn7+—Sn12+ that radiate at the 13.5 nm wavelength that is considered to be optimum for lithography because it matches the high reflectance band of Mo—Si multilayer mirrors.
A very effective method of plasma production for this purpose is the Star Pinch photon source, described in U.S. Pat. No. 6,567,499, issued May 20, 2003 to McGeoch, and U.S. Pat. No. 6,728,337, issued Apr. 27, 2004 to McGeoch. In that device, multiple beams of an ionized working gas are accelerated to a central location, with partial neutralization so that space charge does not build up and prevent the accumulation of material. This phase of operation positions the working material within a small volume at the center of the discharge chamber. An electric pulse is then applied between two groups of the ion beam sources with the result that the central plasma is compressed and heated to the density and temperature necessary for efficient radiation at 13.5 nm. Using xenon or tin as the radiating species, the optimum density and temperature are in the approximate range of 1018–1020 electrons cm−3 and 30 eV respectively. The Star Pinch photon source has operated at a repetition frequency of 2 kHz with 10 J delivered to the plasma on each pulse. Either a pure xenon plasma, or a plasma comprising a noble gas with a partial pressure of tin vapor, has been used in the Star Pinch photon source to generate a high power 13.5 nm photon beam with electrical efficiency of up to 1%, defined as radiated energy into 2π steradians within a 2% band at 13.5 nm divided by electrical input energy.
The Star Pinch photon source is especially suited to long life generation of extreme ultraviolet radiation because it generates the hot EUV-radiating plasma at a much larger distance from the discharge electrodes than other commonly used plasma generation devices, such as the Z-Pinch photon source disclosed in U.S. Pat. No. 5,504,795, issued Apr. 2, 1996 to McGeoch, the dense plasma focus source disclosed in U.S. Pat. No. 6,064,072, issued May 16, 2000 to Partlo et al, or the capillary discharge source disclosed in U.S. Pat. No. 6,061,241, issued Feb. 29, 2000 to Silfvast et al. Large separation between the plasma and the wall is necessary because the plasma exhaust contains energetic ions (300 eV to 1 keV) that erode the wall by sputtering. Using separations of 9 mm to 25 mm in the Star Pinch photon source, we have demonstrated more than 500 million pulse life, extrapolated from a 100 million pulse test. Increased separation will be possible so that life approaching 10 billion pulses is anticipated, entering the range of commercial viability for high volume semiconductor lithography.
For the process of EUV lithography, because of the limited etendue of the reflective projection optics, it is desirable to have a source volume as small as possible and preferably less than a sphere of 1 mm diameter. If the Star Pinch photon source, generates a plasma of length 2 mm (but sub-1 mm diameter), the utilization of radiation will, in this example, only be 50%. The length of the hot plasma generated in the Star Pinch photon source, is related to the diameter of the cooler (preceding) central plasma formed by the intersecting partially neutralized ion beams. Using xenon or tin (in diluent gas) as the working gas, the plasma size has been measured to lie in the range of 2.4–4.0 mm length by 0.6 mm diameter, defined as full width half maximum (FWHM) measurements. By careful design of these beams, the working material may be placed within a smaller volume with the consequence that 13.5 nm emission radiates from a smaller volume. However, there are limits to the amount of size reduction that can be achieved.
It is desirable therefore to provide improved methods and apparatus for generating extreme ultraviolet or soft X-ray photons.